Whereas the initial expense of producing radio frequency power with klystrons is less than that of producing it by combining the outputs of a number of solid state amplifiers, the long term expenses are greater because of the klystron's relatively short life. Furthermore, when a klystron fails, all radio frequency power is lost, but when even a few solid state amplifiers fail, only a portion of the radio frequency power is lost.
One advantageous way of combining the output power of a large number of solid state amplifiers is to operate a number of them in parallel in each of a plurality of modules and to apply the radio frequency outputs of the modules to a radial combiner such as described in an article by Bobby J. Sanders entitled "Radial Combiner Runs Circles Around Hybrids" appearing at pages 55-58 of the November 1980 issue of Microwaves and also in an article by Stephen J. Fote et al entitled "60 Way Radial Combiner Uses No Isolators" appearing at pages 96-100 and 118 of the July 1984 issue of Microwaves. The radial combiner is comprised of spaced circular conductive plates forming a radial waveguide that conducts radio frequency energy introduced at its circumference to an output electrode at the center of one of the plates. The outputs of the modules are coupled to points around the circumference of these plates so that the sum of their outputs appears at the electrode referred to.
Cooling of the modules is accomplished as follows. Electronic components including the solid state amplifiers are mounted on one side of a base of a tray, and passageways for cooling air are mounted on the other. The design has been such that the thickness of a module at the point where it is coupled to the circumferential edge of the radial combiner is the sum of the thickness of the side of the module containing the electrical components and the thickness of the cooling air passageways.
The total output power is, of course, proportional to the product of the number of modules coupled to the radial combiner and the number of solid state amplifiers contained in each module. The number of modules that can be coupled to the circumference of the radial combiner depends on their thickness. Since the length of the circumference of the radial combiner depends on the radio frequency involved, there is a limit to the number of modules of any given thickness that may be used. The number of amplifiers that can be used in a module is limited by the tolerance requirements for circuits that couple radio frequency energy to them.
Operating potential for the amplifiers is generally provided by storage capacitors that are mounted on the same printed circuit board as the amplifiers. In some designs they are mounted between the amplifiers so as to increase the length and thus the power losses of the serial splitter and serial combiner that respectively couple power to and from them. In other designs, the storage capacitors are mounted on top of the respective amplifiers so that they have to be unsoldered and resoldered when an amplifier is being replaced.